A very, very good question. I need to test whether the :encoding(utf8) directive works and I feel that trying it out is more reliable than just checking the perl version.
use strict; use warnings; print "Just Another Perl Hacker\n";
Comment on Re^2: an ever-available file for opening
Hm. I'm not sure that your justification stands up to scrutiny, but if you must, how about:
my $utfData = "\x...";
open UTF, '<:encoding(utf8)', \$utfData or die 'Must be pre-5,8';
die 'Bad stuff happened' unless <UTF> eq 'Whatever you expect';